Field of the Invention
The present invention relates to an object information acquisition apparatus, an object information acquisition method, and a non-transitory computer-readable medium. More specifically, the present invention relates to a technique of acquiring object information by transmitting an acoustic wave to an object and receiving a reflected wave reflected from inside the object.
Description of the Related Art
In the case where image data is acquired by ultrasonic diagnostic apparatuses (i.e., object information acquisition apparatuses) using a pulse-echo method, the depth-direction spatial resolution can be generally expressed as (nλ)/2, where λ denotes the wavelength of an ultrasonic wave and n denotes the number of transmitted waves. For example, in the case where two ultrasonic waves each having a center frequency of 12 MHz are transmitted, the depth-direction spatial resolution is approximately 0.13 mm.
The pulse-echo method will be described. First, an ultrasonic wave pulse (acoustic wave pulse) is transmitted to an object. The ultrasonic wave pulse is then reflected because of a difference in acoustic impedance inside the object, and a reflected wave returns. The reflected wave is received, and image data is generated using the reception signal of the reflected wave. Typically, the envelope of the reception signal is obtained, and the obtained envelope is converted into brightness values. In this way, image data is generated. By repeatedly performing transmission and reception of an ultrasonic wave in a plurality of directions or to a plurality of positions inside the object, pieces of brightness information can be acquired for a plurality of scan lines along the directions in which the ultrasonic wave is transmitted and received. The pieces of brightness information for the plurality of scan lines are then arranged. In this way, the inside of the object can be visualized.
In general, ultrasonic diagnostic apparatuses use a plurality of transducer elements each configured to perform conversion between an ultrasonic wave and an electric signal, and shift waveforms of signals of the individual transducer elements with respect to time. In this way, focusing is achieved inside the object during transmission and reception.
As described above, the pulse-echo method can achieve a depth-direction spatial resolution of approximately 0.13 mm. However, a higher spatial resolution is desired. For example, if more detailed observation of the layered structure of the blood vessel wall of the carotid artery becomes available, such observation may contribute to early detection of atherosclerosis or the like.
Hirofumi Taki, Kousuke Taki, Takuya Sakamoto, Makoto Yamakawa, Tsuyoshi Shiina, and Toru Sato, Conf Proc IEEE Eng Med Biol Soc. 2010; 1: 5298-5301 discloses a result obtained by imaging of the layered structure of the blood vessel wall by performing frequency domain interferometry (hereinafter, referred to as FDI method) and the Capon method which is a type of adaptive signal processing. By performing the FDI method and the Capon method for reception signals, the depth-direction (i.e., scan-line-direction) spatial resolution can be further improved. However, it is considered that a plurality of reflective layers may exist within a depth-direction range (i.e., processing range) of the signal sectioned for FDI processing. Also, it is likely that a plurality of reflected waves reflected from reflective layers located close to one another have a high correlation. It is known that if adaptive signal processing, such as the Capon method, is applied to such reception signals of the plurality of reflected waves having a high correlation without taking any additional measures, unexpected effects such as cancellation of a desired signal occur. In order to reduce the influence of signals having such a correlation (i.e., correlated interference waves), a frequency averaging technique is additionally used. In this way, the FDI method and the Capon method can be applied to reception signals of reflected waves.
In the case of using a frequency averaging technique for reception signals of acoustic waves having a wide frequency band, such as pulse waves, whitening is performed on the reception signals using a reference signal. Japanese Patent Laid-Open No. 2010-183979 discloses an apparatus that combines a plurality of criterion signals for forming a reference signal together at a predetermined interpolation ratio and uses the resulting signal (i.e., computation reference signal) as the reference signal.
As described above, adaptive signal processing in which the FDI method is employed (hereinafter, referred to as FDI-employed adaptive signal processing) uses a reference signal. As the waveform of this reference signal becomes closer to that of an actually obtained reflected wave, a higher spatial resolution is achieved by the FDI-employed adaptive signal processing.
However, the waveform of an acoustic wave pulse transmitted to an object changes depending on a position (i.e., reflection position) which the acoustic wave pulse reaches. In particular, the waveform of the transmitted acoustic wave pulse may change at positions of different depths. For this reason, there may be cases where a sufficiently high spatial resolution is not achieved by the FDI-employed adaptive signal processing.